The Nameless One. Boy howdy, normally he is only around universal level in terms of firepower. However, his immortality makes him damn near impossible to kill even in the context of Planescape: Torment. He can only be killed in an extremely specific plane that exists outside of the multiverse, and even then, you need something truly ridiculous like existential self-denial erasure that warps reality through sheer willpower by retroactively denying that you ever existed or a weapon forged by Coaxmetal. The same person who provides the weapons for the Blood War, and who is a threat to the multiverse due to his connection to entropy. Even nerfed? His immortality is that crazy.